Tony Blair will this week admit that Britain's future in the European Union is
no longer certain.

The former Labour Prime Minister and passionate europhile will warn that Britain will face disaster if it leave the union.

In a bid to thwart a rising tide of euroscepticism in the country, Mr Blair will deliver a speech to the lobby group Business in New Europe on Wednesday warning that Britain will lose power and influence in the world if it leaves.

A source close to the former Leader told The Observer: "Whereas the postwar argument for Europe was about peace versus war, he will make the point that the 21st-century case for Europe is about power versus irrelevance."

Mr Blair, who has amassed a fortune from public speaking and lucrative advisory jobs since leaving office five years ago, will seek to persuade Britain's they would lose out if Britain exited the block of 27 countries.

He will highlight how around half of Britain's trade is with Europe and that a similar proportion of foreign direct investment into the UK comes from the continent.

At the start of his term in Downing Street Mr Blair was a zealous supporter of the British entry into the European single currency. It was reported that he was very enthusiastic about becoming the first President of the European Council in 2009. The continent's leaders chose to appoint Herman Van Rumpoy, a low-profile Belgian.

Mr Blair will call on Europe's leaders to devise a "grand plan" setting out the union's purpose in the 21st century. "He will say that of course you have to get the politics and economics aligned but this has to be part of a grand plan not a series of incremental changes," the source said.